Positrons


ALPHA derives its positrons from a radioactive beta-decay source containing an isotope of sodium, Na-22. This isotope, which has a conveniently long half-life of about 2.6 years, emits positrons with a large spread of kinetic energies up to about 545 keV. Such energetic positrons cannot easily be applied for antihydrogen production, so that ALPHA uses a well established technique to produce a low energy (eV) beam of positrons in vacuum.

Positrons implanted into solid material typically have a lifetime less than one nanosecond, a thousand millionth of a second. However, during that brief time most will slow down by a variety of energy loss processes to reach kinetic energies close to those characteristic of the temperature of the solid. This process is termed moderation, as the positron’s kinetic energy is lowered, or moderated. Whilst most of the positrons penetrate deep into the bulk of the material and annihilate there, about 1% stop close enough to the surface that they can diffuse back to it before they annihilate. Incredibly, most of the positrons which reach the surface are emitted into vacuum at low energy, and can be readily formed into a beam and transported, typically using magnetic guiding fields. ALPHA uses a solid film of condensed neon as its moderator; this is one of the most efficient positron moderators.

The moderator and beam-line are shown on the left of the figure. The sodium-22 source is mounted on a coldhead which is cooled to about 6 degrees Kelvin, and the neon is plated directly on top by admitting gas to the vacuum chamber. A beam of several million positrons per second is available from a 75 mCi (or 2.7 GBq; i.e. about 2.7 x 109 disintegrations per second) source.

The beam is guided by the coils and solenoids shown in the figure. To make antihydrogen we need to trap and manipulate the positrons, which presents a problem since they are emitted in a beta-decay and are not timed. Thus, ALPHA uses a technique based upon buffer gas cooling first developed by Cliff Surko and co-workers. Here an elongated charged particle trap is erected (it is a version of the Penning trap often called the Penning-Malmberg trap; in 3 stages (see figure). Gas is admitted into the first stage, which is at a pressure of about one millionth of an atmosphere. The positron beam passes through the gas and about 20- 30% of them lose energy in a collision and are then captured. They then pass through the second and third stages of the trap, where they again lose energy, eventually ending up in the third stage at a low pressure where they have a lifetime against annihilation of around 100 s. Up to 100 million positrons can be accumulated in such a device in 2-3 minutes.

The cooling gas is ordinary molecular nitrogen, N2, which has been found to be the most efficient gasfor positron capture. This molecule has, luckily, a prominent positron excitation transition (e+ + N2 → e+ + N2*) which involves the positron losing about 9 eV of kinetic energy and which competes effectively with the positron loss channel of positronium (chemical symbol, Ps) formation (e+ + N2 → Ps + N+2).

Several million positrons per second are captured by ALPHA’s accumulator and their number and density quickly become such that a so-called non-neutral plasma is formed. This plasma rotates around the magnetic field axis with a well-defined frequency.This effect can be put to good use as deliberately making the plasma rotate faster causes it to contract adially, due to the conservation of angular momentum. The torque required to do this is provided by an electric field set up using one of the trap cylindrical electrodes which is segmented in six sectors. Applying a sinusoidal voltage appropriately phase shifted for each segments creates a field which appears to rotate as far as the plasma is concerned and with the frequency of the voltage set above the natural rotation frequency, the plasma shrinks. This technique is often referred to as the “rotating wall” method.

Once the positron plasma has the desired number and size, it is ejected from the accumulator and crosses over to the main ALPHA apparatus, where it is recaptured. Since we can control the timing of the ejection accurately, the recapture is easily done using a voltage pulsing system. The magnetic field in this region is around 1 Tesla and so the positrons lose energy quickly by the emission of cyclotron radiation and try to come into thermal equilibrium with their surroundings. Once ALPHA has its positrons here, it can further tailor the plasma to suit the particulars of the experiment underway. This may include further rotating wall compression, stacking of more positron pulses from the accumulator – or perhaps even cutting the number – and shuffling them between Penning traps in the region of the ALPHA main trapping system and its neutral atom trap arrangement.

The ALPHA Penning trap electrodes are held at a temperature of around 7-8 degrees Kelvin, though as yet, our positrons do not reach this temperature. Fortunately, the gas pressure in the cryogenic environment of the trap is very low, such that the positrons have long lifetimes against annihilation. This allows us to actively cool them using a technique borrowed from cold atom physics (and elsewhere) known as evaporative cooling. In our case we lower one side of the electrical well holding the antiparticles. This allows the more energetic to escape, with those that remain coming into thermal equilibrium at a lower temperature by colliding with one another. Doing this several times allows us to gradually cool some positrons to temperatures in the range of several 10’s of degree Kelvin. The positron cloud is then ready to be used for antihydrogen production.

 

Figure 1. Schematic illustration of the ALPHA positron accumulator. The arrangement is on the left is a so-called coldhead, the inner part of which is cooled to 6 degrees Kelvin and which holds the source with the neon moderator plated onto it. The low energy beam is transported at an energy of about 80 eV using the 300 Gauss guiding fields into the 3-stage accumulator, which is situated on the left inside a 1,500 Gauss (0.15 Tesla) solenoid. The lower panel is a representation of the three stages of electrical well into which the positrons are accumulated. The gas is pumped out of the device before the positrons are transferred to the main ALPHA system.